Abstract
Abstract. Knowledge of sea-ice thickness and volume depends on freeboard observations from satellite altimeters and in turn on information of snow mass and sea-ice density required for the freeboard-to-thickness conversion. These parameters, especially sea-ice density, are usually based on climatologies constructed from in situ observations made in the 1980s and earlier while contemporary and representative measurements are lacking. Our aim with this paper is to derive updated sea-ice bulk density estimates suitable for the present Arctic sea-ice cover and a range of ice types to reduce uncertainties in sea-ice thickness remote sensing. Our sea-ice density measurements are based on over 3000 km of high-resolution collocated airborne sea-ice and snow thickness and freeboard measurements in the western Arctic Ocean in 2017 and 2019. Sea-ice bulk density is derived assuming isostatic equilibrium for different ice types. Our results show higher average bulk densities for both first-year ice (FYI) and especially multi-year ice (MYI) compared to previous studies. In addition, we find a small difference between deformed and possibly unconsolidated FYI and younger MYI. We find a negative-exponential relationship between sea-ice bulk density and sea-ice freeboard and apply this parameterisation to one winter of monthly gridded CryoSat-2 sea-ice freeboard data. We discuss the suitability and the impact of the derived FYI and MYI bulk densities for sea-ice thickness retrievals and the uncertainty related to the indirect method of measuring sea-ice bulk density. The results suggest that retrieval algorithms be adapted to changes in sea-ice density and highlight the need of future studies to evaluate the impact of density parameterisation on the full sea-ice thickness data record.
Highlights
Sea ice affects the heat, moisture, and energy exchange between the ocean and the atmosphere, monitoring the state of sea ice is crucial for understanding the current climate, how it may evolve, and what its impact may be (e.g., Stroeve and Notz, 2018)
The unique, collocated, multi-sensor measurements of the Arctic sea ice from the Alfred Wegener Institute’s 80 (AWI) IceBird campaigns allow us to observe sea-ice thickness, freeboard, and snow depth in high-resolution on regional scales, and for the first time to estimate sea-ice bulk densities of different ice types from airborne measurements
Despite measuring the sea-ice bulk density 385 indirectly by deriving it from other measurements, we are able to capture the effects of deformed ice on first-year ice (FYI) bulk density
Summary
Sea ice affects the heat, moisture, and energy exchange between the ocean and the atmosphere, monitoring the state of sea ice is crucial for understanding the current climate, how it may evolve, and what its impact may be (e.g., Stroeve and Notz, 2018). Observing sea-ice thickness and volume over decadal periods relies on freeboard measurements by satellite laser 20 and radar altimeters. The conversion of freeboard to sea-ice thickness requires information of snow mass as well as the density of the sea-ice layer. Observations of both input parameters are sparse and the unknown spatial and temporal variability and trends of snow mass and sea-ice density directly translate into the uncertainty of the sea-ice thickness data record (Giles et al, 2007; Kwok, 2010; Zygmuntowska et al, 2014).
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